Probably just habbit then I would have though obviously hasnt got enough to concentrate on in walk and obviously halt!! I would try doing leg yield etc in walk to get him thinking in the walk and concentrate on what you are asking rather then chimping on the bit! good luck
He's a youngster Hel.. could just be doing it for something to do/play with.
Razz has only recently stopped playing with his when he's tied up on the yard tacked up.